In the manufacture of microcircuit dies, chemical/mechanical polishing (CMP) is used to provide smooth topographies of the semiconductor wafers for subsequent lithography and material deposition. Briefly, the CMP process involves holding and rotating a thin, reasonably flat, semiconductor wafer while pressing the wafer against a rotating polishing surface or platen. The semiconductor wafer is held in a carrier that has a carrier ring about its periphery to restrain the wafer to a position under the carrier. The polishing surface is wetted by a chemical slurry, under controlled chemical, pressure, and temperature conditions. The chemical slurry contains a polishing agent, such as alumina or silica, which is used as the abrasive material. Additionally, the slurry contains selected chemicals which etch or oxidize specific surfaces of the wafer during processing. The combination of mechanical and chemical removal of material during polishing results in superior planarization of the polished surface.
During polishing, a downward vertical force is applied to the carrier head through a gimbal by a load cell mounted on the carrier head drive shaft. The gimbal is essential in this design to allow the carrier head to conform to undulations of the polishing platen. Of course, the inclusion of a load cell on the carrier head drive shaft adds mass to a rotating system, thus complicating system balance. The total force applied by the load cell is generally distributed over the area of the wafer by the gimbal.
A polishing pad that rests on the surface of the polishing platen receives and holds the chemical slurry during polishing. As the platen and pad are rotated in contact with the wafer, the flexible polishing pad tends to develop a ripple near the edge of the carrier head. Because of the extremely small tolerances necessary in semiconductor manufacture, it is important to maintain the planarity of the wafer. In order to avoid rounding the edges of the wafer through contact with the ripple, the carrier ring may be extended toward the polishing pad with pneumatic pressure to force the ripple outward toward the circumference of the carrier ring and away from the wafer. This system is generally complex and expensive to maintain while less accurate than is desired for high-precision semiconductor manufacture.
Accordingly, what is needed in the art is a simpler apparatus and method to apply the forces necessary for chemical/mechanical polishing of semiconductor wafers.